The present invention is directed to a threaded plug for use with a hammer union, and an assembly thereof with a hammer union, particularly for use with hammer unions used in high pressure piping for hydraulic fracturing of subterranean formations (or “fracking”).
Natural gas is found in the pores of shale or coal, and it escapes through natural fractures. Surface deposits are relatively easy to recover with simple drilling. However, for the deepest and richest deposits, high pressure reduces the number of naturally occurring fractures and the rock's permeability is insufficient to extract much gas. Shale beds are often less than a hundred meters thick, so fracking boreholes are usually drilled horizontally to extend through as much of the bed lengthwise as possible. The boreholes are sealed with pipe. High pressure water, sometimes as high as 10,000 psi or greater, is then sent down this pipe. This system acts just like a hydraulic ram, where great force can be applied over a large area by introducing high pressure into the ram from a small entry point. This force splits the shale apart, creating numerous small fractures. Once done, extraction wells going straight down into the shale bed are far more productive, as the gas now has many free escape routes.
The hydraulic fracturing process requires many connections to be made from hydraulic fracturing pumping trucks (frac truck) to piping manifolds that connect to the well bore. The separate sections of pipe are connected by means of hammer unions. Hammer unions are designed to join separate sections of pipe without need of connecting flanges. Hammer unions are less expensive than flanged connections, and can generally be assembled more quickly.
Hammer unions are comprised of a male sub (or nut end), a female sub (or threaded end), and a threaded hammer nut which engages an annular flange on the male sub, and the screw threads on the female sub, to compress an internal seal assembly to prevent leakage and maintain a strong seal between the respective pipes. The seal assembly is generally an O-ring or lip seal that is inserted into the thread end of the hammer union and ultimately compressed.
Given the extremely high pressures employed during hydraulic fracturing, it is extremely important for the joint of the respective pipes to be sufficiently strong to counteract such pressures. The lack of a reliable high-pressure metal-to-metal seal (as is the case with a flange connection) has been cause for concern in the industry, as hammer union joints are more prone to high pressure failure than flange connections. One cause of possible failure of the hammer union seal is thread damage on the connecting surfaces, due possibly to repeated use and/or misuse and excessive handling and storage of the hammer union fitting, as well as damage to the sealing surfaces of the male and female subs. Avoidance of damage to the threaded nut and sealing surfaces reduces the possibility of injury to workers due to high pressure failure of the hammer union seal, as well as contamination of the environment due to leakage of hydraulic fracturing fluids.
A few types of hammer union plug protectors are commercially available. However, such commercially-available protectors are believed to be deficient from the standpoint of the overall protection provided.